1. Field of the Invention
The present invention relates to an analog trunk circuit used in a telephone switching system, and more particularly a programmable analog trunk circuit which may serve up multifunction under software control.
2. Description of the Related Art
Generally, the trunk circuit (TRK) interfaces the switching system with the central office line so as to form a communication loop by seizing the central office line. TRK circuits may be usually divided into two groups, namely, analog and digital type according to the method of transmitting signals. In addition, the analog TRK circuits may be further divided into loop TRK, ground start TRK, DID (Direct Inward Dialing tie line) TRK, E&M (Ear & Mouth tie line) TRK, ring down tie line TRK, and both way tie line TRK circuits according to their functions.
Most of the switching systems are in the form of a cabinet having a plurality of shelves to respectively mount a control board, switching board, subscriber board, trunk board, etc. As for the TRK board, it is preferably designed to accommodate various daughter boards, each serving as a particular TRK circuit. For example, the conventional analog TRK board, as shown in FIG. 1, includes a plurality of communication ports to respectively connect with loop TRK circuit, ground start TRK circuit, DID TRK circuit, E&M TRK circuit, ring down TRK circuit and both way TRK circuit board.
As described above, such conventional switching system requires a plurality of daughter boards having respective functions to meet various trunk circuits, thus increasing the cost of maintaining the system. Moreover, in order to replace a daughter board with another for accommodating a new operational environment, the corresponding shelf should first be removed from the cabinet, thereby causing much inconvenience.